1. Field of the Invention
The present invention relates to a high-frequency amplifier applicable to the front-end section of a radio communication apparatus such as a television tuner and also relates to the radio communication apparatus employing the high-frequency amplifier.
2. Description of the Related Art
In most cases, an LC resonator is used as a load borne by a high-frequency amplifier employed in a receiver in which only a desired frequency is selected by imposing band limits on RFs (radio frequencies) of received signals.
FIG. 1 is a circuit diagram showing a typical configuration of a high-frequency amplifier 1 employed in a front-end circuit to serve as an amplifier which includes a parallel LC resonator used as a load circuit 3 borne by a gm amplifier 2 employed in the high-frequency amplifier 1.
That is to say, as shown in the circuit diagram, the high-frequency amplifier 1 employs the gm amplifier 2, the load circuit 3 as well as output terminals TO1 and TO2.
The load circuit 3 is provided between the output node of the gm amplifier 2 and the output terminals TO1 and TO2.
To put it in more detail, a node ND1 of the load circuit 3 is connected between the output node of the gm amplifier 2 and the output terminal TO1 whereas a node ND2 of the load circuit 3 is connected between the output terminal TO2 and a reference electric potential VSS.
The load circuit 3 employs a variable-capacitance capacitor C, an inductor L and a resistor R which is used as a resonant impedance element.
The variable-capacitance capacitor C, the inductor L and the resistor R are connected in parallel between the nodes ND1 and ND2 to form the load circuit 3.
Typically, the load circuit 3 has a configuration in which the capacitance of the variable-capacitance capacitor C is varied in order to change the frequency band of the received input signal, and the high-frequency amplifier 1 is compensated for a gain variation accompanying the change of the frequency band of the received input signal.
FIG. 2 is a diagram showing a graph indicating dependence of the gain of the high-frequency amplifier 1 on the frequency of the input signal. In the diagram of FIG. 2, the horizontal axis represents the frequency of the input signal whereas the vertical axis represents the gain of the high-frequency amplifier 1.
The resonant impedance Ro of the parallel LC resonator serving as the load borne by the gm amplifier 2 is an impedance at the resonant time of the parallel LC resonator. The resonant impedance Ro of the parallel LC resonator serving as the load rises as the frequency of the input signal increases. This is because the resonant impedance Ro of the parallel LC resonator is determined by ωLQ. The gain of the high-frequency amplifier 1 is expressed in terms of a transconductance gm of the gm amplifier 2 and the resonance impedance Ro in accordance with the following equation:
                              Vout          Vin                =                  gm          ·                      Ro            ⁡                          (              ω              )                                                          (        1        )            
As a result, the high-frequency amplifier 1 shown in the circuit diagram of FIG. 1 has a gain with a frequency-dependence characteristic like the one shown in the diagram of FIG. 2. For more information, the reader is advised to refer to documents such as Japanese Patent Laid-Open No. 2008-160660.